1. Field of the Invention
This invention relates to firing circuits and more particularly to an SCR firing circuit that is useful in industrial control applications.
2. Description of Related Art
Firing circuits employing silicon controlled rectifiers or SCR's are used for powering equipment or other loads such as variable frequency or electromagnetic feeders. Typically, a microcontroller is employed for sending an appropriate signal to an SCR for powering the load. SCR's are turned on to their conducting state by applying a trigger pulse to their gate connection at some time during a half cycle of the power source line frequency. When triggered, the SCR continues to conduct until the current passing through it drops below its holding current value. By delaying the point in time of the trigger pulse, relative to the applied half cycle of power during which the device is triggered, the average power applied to a connected load may be changed for control purposes.
Various schemes have been devised and used to trigger SCR's but, other than directly connecting the gate to the anode, in which case the SCR becomes just a diode, there is always some delay from the zero cross point of the applied voltage wave form to the point where the SCR turns on, depending on the circuitry employed, such that it is not possible to obtain the maximum power available from the source. It may be difficult to fire the SCR close enough to the zero cross of the voltage waveform because of the time required to have sufficient power available to fire the SCR. It is therefore desirable to have an SCR firing circuit which fires closer to the zero cross point of the voltage waveform.
It is also desirable to electrically isolate the logic circuitry for control from the output power of the device, particularly if very high voltages are involved. Opto-couplers are used to isolate a low control voltage from a high power voltage. However, use of a single opto-coupler may not be strong enough to isolate higher power voltages. Other prior art devices have used a double stacked opto-coupler arrangement. However, such a double stacked arrangement may be limited to isolating the low control voltage from a supply of approximately 240 Volts. It is therefore desirable to provide an arrangement of opto-couplers that may isolate a low control voltage from rather high power voltages such as 380 to 600 Volts at 50 or 60 Hz. It is further desirable to provide a microcontrolled gate circuit capable of firing a wide variety of voltages.
Because of feed material characteristics on vibratory feeders or conveyors, it is often desirable to operate such equipment at low frequencies with higher strokes. When such equipment is powered by an electromagnet, it is cost effective and convenient to synthesize the low operating frequency from the 60 Hz or 50 Hz power source, by skipping an appropriate number of half cycles of the line frequency. For example, from a 60 Hz power line, frequencies that are even sub multiples of 120 pulses per second (7200 VPM) can be obtained. Dividing by 2 gives 60 Hz, dividing by 3 gives 40 Hz, by 4 gives 30 Hz, and so on. Since the actual power "on time" at any given frequency is limited to that of a half cycle at 60 Hz, it is desirable to capture as much of the available wave form as possible to maximize the output power. Therefore, it is desirable to minimize the amount of time delay from the zero voltage cross of the conducting wave form to the time the SCR turns on.
Moreover, feeders such as these may operate at power line voltages of up to 600 Volts AC. While SCR's are available with reverse voltage ratings of 1200 Volts or better, because of the requirement for controlling highly inductive, high voltage loads, generally pulse transformers are used for firing SCR's. However, pulse transformers may result in an undesirably long delay for the firing of the SCR and thus not enough output power may be obtained. In addition to the cost of pulse transformers, it may be more difficult to interface the control logic to the pulse transformer, and thus a more costly a solution. It is therefore desirable to have a firing circuit which has minimal delay in firing an SCR and which is also cost effective.